Distribution, movements and feeding ecology of killer whales (Orcinus orca) in tropical waters, with particular reference to the Southwest Indian Ocean

Abstract

Killer whales (Orcinus orca) are cosmopolitan apex predators, distributed from the poles to the tropics. Most knowledge of the species comes from high latitudes, where long-term studies have described populations with distinct morphology, diet, genetics, behaviour and acoustic repertoires. In contrast, their ecology in tropical and sub-tropical waters remains poorly known. This thesis aimed to address data gaps in the large-scale movements and feeding ecology of killer whales in warm waters. Stable isotope methods were applied to analyse annual growth layers of teeth from low-latitude specimens around the world. First, pre-analysis dentine treatments were compared to establish best-practice protocols. Second, carbon and oxygen isotopes from tooth hydroxyapatite were analysed and not only showed that low-latitude killer whales differed significantly in isotope values from published high-latitude specimens, but also revealed that they likely never undertook large-scale latitudinal movements to cold waters. Third, carbon and nitrogen isotopes from the collagen of the same individuals’ growth layers were analysed to determine the weaning age of individuals from the tropics and sub-tropics and to explore their isotopic niches. Weaning was estimated to be complete at four years, and individuals’ trophic level did not change significantly throughout their adult lives. There was marked inter-individual variability in trophic level and foraging habitat, both within and across oceans. Finally, the ecology of the tropical Southwest Indian Ocean population was examined. Prey predictions from stable isotopes and reported predation events both suggested a broad, generalist diet. A 25-year dataset of opportunistic sightings and photo-identification records indicated year-round presence, small-scale movements and site-fidelity across the region. Despite limitations associated with small sample sizes, this thesis supports the existence of distinct tropical killer whale populations and underscores the need for expanded research in low latitudes to better describe the connectivity and population structure of this species globally.

Date of Award	30 Jun 2026
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Sascha Hooker (Supervisor) & Phil Hammond (Supervisor)